1. Field of the Invention
The invention relates to a metal foil electroformed from a master pattern and, more particularly, a metal foil serving as packing for sheet-guiding cylinders and/or drums on rotary printing machines, the metal foil having one flat face, and an opposite face with a textured surface structure. The invention also relates to the master pattern and a method of forming the metal foil and the master pattern.
2. Description of the Related Art
Heretofore known metal foils of this general type, as described in German Published Prosecuted Application (DE-AS) 26 05 330, are preferably formed of solid nickel, and possess a surface structure or texture corresponding substantially to a glass beaded fabric which has also become known heretofore for the same purpose. This results from the fact that electroforming (previously known as galvanoplastic molding) of metal foils is effected from a negative pattern which, for its part, is molded from a positive master pattern made up of a carrier or support foil having a layer of rubber applied thereto, with glass balls embedded in the rubber and partially protruding from the surface thereof. An advantage of these known metal foils is that the surface structure or texture is largely reproducible. This is vital for speedy and efficient use in the printing process, for example, for the control-free exchange of a damaged metal foil for a new one. A disadvantage of these known metal foils, however, is that no optimal surface topography is available with respect to various operating requirements.
The same disadvantages are apparent in another heretofore known solution (European Patent 17 776), wherein a sheet-guiding foil, as packing for impression cylinders of rotary printing machines for perfector printing, is flat on one surface, and is provided with spherical calottes of equal height in a statistically uniform distribution, and wherein the foil is formed of a carrier or support layer and a covering layer, the carrier or support layer being formed of nickel or plastic material with a high modulus of elasticity, e.g. polyamide or PVC, a covering layer being applied in the form of a thin chromium layer to the surface on which the spherical calottes are formed so as to compensate for micro-roughness. This compensation for the micro-roughness does not change the contemplated, very even spherical calotte or shell topography of the surface.
With regard to reaching a compromise concerning roughness, German Patent 12 58 873 proposes surface structures or textures for an impression cylinder and for an aluminum foil assigned thereto, respectively, which is formed with a chromium surface having a roughness (RMS) of between 2 and 7.5 mu. By this means, two marginal conditions in the compromise should be optimally fulfilled, namely, that the roughness on the one hand, is sufficient to bring about an asserted given ink-repellant effect, for example to impede slurring or smearing of the rear side of the freshly-printed sheet during second-side or perfector printing, and, on the other hand, that the roughness is as low as possible, in order to ensure the optimal support or contact surface area for the bearing surface of the sheet. On the one hand, as has been found, this compromise is not optimally achieved. On the other hand, this solution has the disadvantage that it is not reproducible with regard to the surface structure or texture. Even if the dimensioning of the roughness (taken over the whole foil at the corresponding average) is reproduced with acceptable tolerance, the surface structure or texture of each foil as a whole again deviates very markedly from the next foil, and each cylinder surface deviates from the next cylinder surface, respectively. The reproducible jet treatment of such thin aluminum foils is also problematic, as is the stability of use thereof. All products previously known in this regard with jet-roughened surfaces accordingly present the same unique characteristics.